Add Etcd v3 protocol support via api gRPC-gateway (#1162)

The only python-etcd3 client working directly via gRPC still supports only a single endpoint, which is not very nice for high-availability.

Since Patroni is already using a heavily hacked version of python-etcd with smart retries and auto-discovery out-of-the-box, I decided to enhance the existing code with limited support of v3 protocol via gRPC-gateway.

Unfortunately, watches via gRPC-gateway requires us to open and keep the second connection to the etcd.

Known limitations:
* The very minimal supported version is 3.0.4. On earlier versions transactions don't work due to bugs in grpc-gateway. Without transactions we can't do atomic operations, i.e. leader locks.
* Watches work only starting from 3.1.0
* Authentication works only starting from 3.3.0
* gRPC-gateway does not support authentication using TLS Common Name. This is because gRPC-proxy terminates TLS from its client so all the clients share a cert of the proxy: https://github.com/etcd-io/etcd/blob/master/Documentation/op-guide/authentication.md#using-tls-common-name

README.rst

@@ -96,7 +96,7 @@ Patroni can be installed with pip:
 
 where dependencies can be either empty, or consist of one or more of the following:
 
-etcd
+etcd or etcd3
     `python-etcd` module in order to use Etcd as DCS
 consul
     `python-consul` module in order to use Consul as DCS
@@ -106,6 +106,8 @@ exhibitor
     `kazoo` module in order to use Exhibitor as DCS (same dependencies as for Zookeeper)
 kubernetes
     `kubernetes` module in order to use Kubernetes as DCS in Patroni
+raft
+    `pysyncobj` module in order to use python Raft implementation as DCS
 aws
     `boto` in order to use AWS callbacks